How do you assign multiple shrinkwrap modifiers to rig muscles?
What I do for muscles rig is that I have 2 Armatures and 2 Meshes:


All muscles as 1 mesh with rig
Character mesh with rig


That means no muscle groups and no clutter in scene and character and only 1 shring-wrap modifier.


The muscle rig bones are parented to character rig with Child Of constraints. This also allows for muscle layering.
The individual muscle jiggle is controlled with 1 vertex group for soft-body goal.
Each muscle has its own shape-key controlled with muscle-rig bones.
Character mesh has only 1 shrink-wrap with vertex-group only for muscle-bulging strength.